Indigenous Herbivores Research Articles

Generalist indigenous herbivores resist alien tree invasion: Rhabdomys pumilio limits establishment of Acacia cyclops

Generalist indigenous herbivores resist alien tree invasion: Rhabdomys pumilio limits establishment of Acacia cyclops

Resilience modes of an ancient mountain valley grassland in South Africa indicated by palaeoenvironmental methods

Grassland ecosystems supporting wildlife and livestock populations have undergone significant transformation in the last millennium. Climate, herbivory, fire, and people are identified as important drivers of ecosystems dynamics; however, grassland resilience has been rarely explored in landscapes with mixed grazing histories. Here we analyse ecosystems states from a South African mountain valley grassland in the last 1250 years using palaeoenvironmental proxies. Our results suggest that a tallgrass phase maintained by climate, people and fire replaced a shortgrass phase driven by indigenous herbivores after ca. 690 cal BP. Furthermore, the tallgrass phase had unpalatable grasses and disturbed soil. We suggest these ecological changes were linked to climate change and arrival of pastoralists in the region. Therefore, our results indicate that human activities may undermine resilience of grasslands and that reversing some changes may be difficult.

When less is more: heterogeneity in grass patch height supports herbivores in counter-intuitive ways

Herbivores are an integral part of the African landscape and have evolved with the vegetation to create the savanna landscape. Managers of these landscapes can benefit from a better understanding of how indigenous herbivores use the landscape to which they are adapted. In this study we observed which patches were frequently utilised, by doing regular monthly road counts, grass height observations and dung counts on selected short grass patches in the Kruger National Park. Smaller-framed impala and blue wildebeest (meso-herbivores) were most regularly seen on these nutritious patches, while from dung deposits it was clear that the even larger-framed buffalo (mega-herbivores) spent time there. This preference can be explained by considering the nutritional needs and food intake of the herbivores. Smaller-framed herbivores seem to be able to satisfy their dietary requirements on the high-quality forage patches, while larger-framed herbivores seem to supplement the quality forage by also spending foraging time on areas of higher grass biomass. From this insight we propose that range management should take herbivore preferences into account and allow herbivores to select and concentrate their foraging on the most nutritious forage. This approach is likely to decrease inputs while allowing animals to maintain or increase production.

Trophic interactions between indigenous and non-indigenous species in Lampedusa Island, Mediterranean Sea

Using stable isotope analysis, we investigated trophic interactions between indigenous benthic taxa and the non-indigenous species (NIS): the green alga Caulerpa cylindracea, the red alga Asparagopsis taxiformis, the crab Percnon gibbesi and the sea hare Aplysia dactylomela. The study was conducted on Lampedusa Island, Mediterranean Sea. We evaluated the trophic positions and isotopic niches of consumers. Using Bayesian mixing models, we quantified the food source contribution to diets of indigenous and non-indigenous herbivores. Isotopic niche of NIS showed no overlap with the ones of indigenous macroinvertebrates and fish. Caulerpa cylindracea provided the largest contribution to the diet of P. gibbesi (0.431–1), while the dietary contribution estimates overlapped considerably for all sources of A. dactylomela and indigenous herbivores. From these results, we conclude that the invasion of C. cylindracea is increasing the diversity of available prey and might facilitate the expansion of other NIS.

Using a botanical garden to assess factors influencing the colonization of exotic woody plants by phyllophagous insects.

The adoption of exotic plants by indigenous herbivores in the region of introduction can be influenced by numerous factors. A botanical garden in Western Siberia was used to test various hypotheses on the adaptation of indigenous phyllophagous insects to exotic plants invasions, focusing on two feeding guilds, external leaf chewers and leaf miners. A total of 150 indigenous and exotic woody plant species were surveyed for insect damage, abundance and species richness. First, exotic woody plants were much less damaged by chewers and leaf miners than native plants, and the leaf miners' species richness was much lower on exotic than native plants. Second, exotic woody plants having a congeneric species in the region of introduction were more damaged by chewers and hosted a more abundant and species-rich community of leaf miners than plants without native congeneric species. Third, damage by chewers significantly increased with the frequency of planting of exotic host plants outside the botanical garden, and leaf miners' abundance and species richness significantly increased with residence time in the garden. Finally, no significant relationship was found between insect damage or abundance and the origin of the exotic plants. Besides the ecological implications of the results, this study also illustrates the potential of botanical gardens to test ecological hypotheses on biological invasions and insect-plant interactions on a large set of plant species.

Unraveling the biogeographic origins of the Eurasian watermilfoil (Myriophyllum spicatum) invasion in North America.

Using phylogeographic analyses to determine the geographic origins of biological invaders is important for identifying environmental adaptations and genetic composition in their native range as well as biocontrol agents among indigenous herbivores. Eurasian watermilfoil (Myriophyllum spicatum) and its hybrid with northern watermilfoil (M. sibiricum) are found throughout the contiguous United States and southern Canada, forming one of the most economically costly aquatic plant invasions in North America, yet the geographic origin of the invasion remains unknown. The objectives of our study included determining the geographic origin of Eurasian watermilfoil in North America as well as the maternal lineage of the hybrids. DNA sequence data from a cpDNA intron and the nrDNA ITS region were compiled for accessions from 110 populations of Eurasian watermilfoil and hybrids from North America and the native range (including Europe, Asia, and Africa). Datasets were analyzed using statistical parsimony and Bayesian phylogenetics to assess the geographic origin of the invasion. The two Eurasian watermilfoil cpDNA haplotypes in North America are also found from China and Korea, but not elsewhere in the native range. These haplotypes did not overlap and were limited in native geographic range. The ovule parent for hybrids can come from either parental lineage, and multiple haplotypes from both parental species were found. The geographic origin of this prolific aquatic plant invasion of North America is in Asia. This provides critical information to better understand the invasion pathway and inform management into the future.

Evaluation of wild herbivore faeces from South Africa as a potential source of hydrolytically active microorganisms.

This study assessed faecal matter from three indigenous South African herbivores—zebra, giraffe and impala—as a potential source for hydrolytically active aerobic and facultatively anaerobic bacteria. Herbivore droppings were collected freshly in a local nature reserve in Pietermaritzburg, South Africa. Soil samples adjacent to faecal collection sites and faeces from a domestic herbivore, the Nguni cow, were included as controls. Hydrolase and dehydrogenase activity in faecal matter and soil samples were measured by the fluorescein diacetate and the triphenyltetrazolium chloride assay. Viable counts and counts for amylase, cellulase, esterase and protease producers were established using plate count agar and solid media containing cellulose, skim milk, starch and Tween 80. Zebra droppings produced the highest hydrolase and dehydrogenase activity. Faecal matter of the three indigenous herbivores generally produced higher hydrolytic activity than Nguni cow faeces and soil controls, thereby confirming that these materials are potential targets for hydrolytic enzyme mining.

Impacts of fire and grazing management on South Africa’s moist highland grasslands: A case study of the Steenkampsberg Plateau, Mpumalanga, South Africa

Background: Grasslands are heavily utilised for livestock agriculture and the resultant degradation through mismanagement contributes to an estimated 60% of this biome being permanently transformed. This study focused on the impact of fire and grazing in moist highland grasslands.Objectives: To determine the contribution of burning frequency and grazing intensity combined (for domestic livestock and indigenous ungulates) on vegetation structure heterogeneity and species diversity.Methods: Eight study sites under different management regimes were sampled over two summers. Vegetation structure characteristics and diversity data were collected monthly within multiple replicates in each study site. A disc pasture meter was used to assess standing biomass. Differences in vegetation structure characteristics, plant community composition and plant species assemblage structure across sites were statistically analysed using analyses of variance, indicator species analyses, multidimensional scaling ordinations and two-way cluster analyses.Results: The combination of heavy grazing and annual burning leads to a distinct plant community dominated by disturbance specialist species. Selective grazing by indigenous herbivores promotes a community of unpalatable species. This study illustrates that fenced indigenous herbivores, even at moderate stocking densities, have a greater detrimental impact on plant diversity and structure than do domestic livestock.Conclusion: Intensive grazing and burning have a detrimental impact on plant species diversity and structure. This also affects resultant palatability for grazing livestock and fenced game. To promote both grazing quality and ecological integrity we recommend a minimum sustainable ‘fodder capacity’ or standing phytomass of 5000 kg per large-animal unit per hectare for domestic livestock in moist highland grasslands.

Utility of a forage-productivity model for predicting herbivore abundance in the eastern Karroo, South Africa, varies among habitats

The question of how many herbivores an area can support for long-term persistence is fundamental to managing wildlife in increasingly fragmented landscapes. Species-specific spatial requirement information is scarce, so many studies have used models to predict carrying capacities for indigenous herbivores based on forage productivity estimates and metabolic requirements of the herbivores. Testing such models against empirically derived observations is, however, rare in the peer-reviewed literature. We modeled predicted relative abundances of the medium- to large-sized herbivore community of a reserve in the semi-arid eastern Karoo, South Africa, following Boshoff et al. (2001, 2002a). We also addressed the overestimation abundance of small species by incorporating an adjustment factor based on the relationship between body size and population metabolic rate. We tested both model's outputs against empirically derived game count data for the same herbivore community which revealed significant differences in species relative abundances predicted by the two models. Habitat-specific regression analyses revealed that incorporating a population metabolism adjustment factor into the model resulted in a generally better fit than the original model. Furthermore, the model performed best for apparently structurally simple habitats. These results support the use of forage-based productivity models to estimate potential abundances of indigenous herbivores.

Native insect herbivory limits population growth rate of a non-native thistle

The influence of native fauna on non-native plant population growth, size, and distribution is not well documented. Previous studies have shown that native insects associated with tall thistle (Cirsium altissimum) also feed on the leaves, stems, and flower heads of the Eurasian congener C. vulgare, thus limiting individual plant performance. In this study, we tested the effects of insect herbivores on the population growth rate of C. vulgare. We experimentally initiated invasions by adding seeds at four unoccupied grassland sites in eastern Nebraska, USA, and recorded plant establishment, survival, and reproduction. Cumulative foliage and floral herbivory reduced C. vulgare seedling density, and prevented almost any reproduction by C. vulgare in half the sites. The matrix model we constructed showed that this herbivory resulted in a reduction of the asymptotic population growth rate (λ), from an 88% annual increase to a 54% annual decline. These results provide strong support for the hypothesis that indigenous herbivores limit population invasion of this non-native plant species into otherwise suitable grassland habitat.

Resources driving landscape-scale distribution patterns of grazers in an African savanna

In order to effectively manage and conserve indigenous herbivores, a good understanding is needed of how resources drive their distribution patterns. This study employed a unique dataset to test a range of ecological theories and hypotheses on free-ranging grazers. Using aerial census data collected over 14 yr across the 2 million ha Kruger National Park (South Africa), this study employs spatial autologistic regression models to explore the spatial relationships that exist between the distribution of eight indigenous grazer species and a set of resource variables. It was found that ecological theories relating to feeding guild, water-dependence, allometric scaling, gut-morphology and vulnerability to predation could explain most of the grazer distribution patterns observed in relation to surface-water, forage quality, forage quantity and habitat openness. All the grazers studied were water-dependent and occurred close to a permanent source of water in the dry season. This was ascribed to the lack of moisture in the diet of grazers during the dry season. Most ruminants' distribution patterns were significantly associated with areas of high forage quality whereas hind-gut fermentors were neutral towards forage quality. Average forage quantity was not a significant predictor of long-term, landscape-scale distribution patterns for any of the grazer species studied. Most small- and medium-bodied grazers preferred open habitats above closed habitats, probably due to higher visibility and a lower predation risk. Large-bodied grazers did not bias their distribution patterns towards open habitats. The way in which grazers distribute themselves with respect to different resources can potentially inform management actions on appropriate scales.

The impact of indigenous ungulate herbivory over five years (2004–2008) on the vegetation of the Little Karoo, South Africa

This study investigated the impact of rainfall and herbivory by indigenous herbivores over five years (2004–2008) on the vegetation of the Sanbona Wildlife Reserve (SWR), a 54 000 ha privately owned conservation area in the Little Karoo. Changes in floristic composition, species richness, aerial plant cover and grazing capacity were determined using drop-point surveys in fenced and open (i.e. grazed) plots in four dominant vegetation types. Annual and seasonal rainfall totals, herbivore stocking rate and herbivore utilisation patterns were also measured. Ordination analyses showed that fenced plots did not develop a fundamentally different successional trajectory from open plots over time. There were also no significant differences in species richness, plant cover and grazing capacity between fenced and open plots in all vegetation types. This lack of indigenous herbivore impact is ascribed to the relatively low stocking rates that were maintained on SWR during most of the study period. Species richness, plant cover and grazing capacity also did not differ significantly between years in all vegetation types except for the highly disturbed Old Land sites. The existing monitoring programme provides a useful tool for assessing the impact of indigenous herbivores and should be supported at local and provincial levels.

Direct and indirect effects of alien insect herbivores on ecological processes and interactions in forests of eastern North America

Alien invasive insects such as gypsy moth, hemlock woolly adelgid, and emerald ash borer continue to disturb the mixed deciduous and hemlock forests of eastern North America by causing wide-scale defoliation, decline and/or mortality of their hosts. Some of the most devastating species are spreading in “defense free space”, causing extensive mortality of hosts that are inherently susceptible, perhaps due to their lack of coevolutionary history with the invader. These disturbances have altered the dynamics of canopy gaps, coarse woody debris, biogeochemical cycling, and ecological interactions among organisms in terrestrial and aquatic systems, with consequent effects on forest composition, structure, and function. Populations of indigenous species specialized to particular habitats and/or host trees are most likely to decrease, while some generalist and opportunistic species may increase in invaded forests, including exotic plants as their facilitation by alien insects sparks an “invasional meltdown”. Although poorly documented, alien insects may induce positive feedback effects on ecological processes and interactions. For example, effects of herbivory on foliar chemistry may indirectly alter tri-trophic interactions of indigenous herbivores on their shared hosts, slow rates of terrestrial nutrient cycling, and decrease productivity of aquatic habitats based on allochthonous inputs. Tactics used to eradicate or suppress alien insects in forests such as insecticide applications, biological control, and silvicultural prescriptions can also have ecological impacts. As alien insects continue to establish and spread in forests of eastern North America, their already pervasive effects on ecological interactions and ecosystem processes will continue to magnify.

A fence line in time demonstrates grazing‐induced vegetation shifts and dynamics in the semiarid Succulent Karoo

The mediterranean ecosystems of the world harbor exceptional biodiversity, and factors such as livestock grazing which may negatively affect biodiversity are of global concern. The mediterranean ecosystems of southern Africa have only sporadically been exposed to high densities of indigenous herbivores and may not be as typically resilient to livestock grazing pressure as the savannah and grassland ecosystems of semiarid Africa. We investigate this issue by assessing the impacts of two contrasting livestock grazing intensities associated with communal and commercial ranching systems on the dwarf succulent shrublands of the Succulent Karoo in Namaqualand, South Africa. We interpret the results in terms of vegetation dynamics and the implications for the current debate on nonequilibrium rangelands. The results demonstrate that the relative extent of plant community divergence between the communal and commercial rangeland has been maintained over the past 10 years, overall and within different plant growth forms. Recruitment of certain palatable shrub species occurred on the heavily grazed communal rangeland, indicating that a further decline in their populations is not inevitable. Recruitment of the unpalatable shrub Galenia africana on the communal rangeland was disproportionately greater than that of the palatable species, reenforcing the dominance of this species. Due to the longevity of woody shrubs, short-term vegetation shifts are small, limiting the potential for the recovery of overgrazed shrublands during periods of low grazing pressure. The shift on the communal rangeland toward annuals and geophytes has increased the variability of forage production and contrasts with the dominance of dwarf shrubs on the commercial rangeland. Shrub-dominated ecosystems create the potential for forage storage and inter-annual transfer of excess production, buffering livestock numbers in the face of a fluctuating environment. Despite the semiarid nature of the area and contrary to the predictions of the nonequilibrium theory, livestock and vegetation in Namaqualand interact strongly, and a close relationship between livestock numbers and primary production persists over the long-term on the communal rangeland.

Do rice water weevils and rice stem borers compete when sharing a host plant?

The rice water weevil (RWW) Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae) is an invasive insect pest of rice Oryza sativa L. in China. Little is known about the interactions of this weevil with indigenous herbivores. In the present study, adult feeding and population density of the weevil, injury level of striped stem borer Chilo suppressalis (Walker) (Lepidoptera: Pyralidae) and pink stem borer Sesamia inferens (Walker) (Lepidoptera: Noctuidae) to rice, as well as growth status of their host plants were surveyed in a rice field located in Southeastern Zhejiang, China, in 2004 with the objective to discover interspecific interactions on the rice. At tillering stage, both adult feeding of the weevil and injury of the stem borers tended to occur on larger tillers (bearing 5 leaves) compared with small tillers (bearing 2~4 leaves), but the insects showed no evident competition with each other. At booting stage, the stem borers caused more withering/dead hearts and the weevil reached a higher density on the plants which had more productive tillers and larger root system; the number of weevils per tiller correlated negatively with the percentage of withering/dead hearts of plants in a hill. These observations indicate that interspecific interactions exist between the rice water weevil and the rice stem borers with negative relations occurring at booting or earlier developmental stages of rice.

Competition and herbivory influence growth and survival of shrubs on old fields: Implications for restoration of renosterveld shrubland

Question: How does competition by grasses, grazing by indigenous large herbivores, and their interaction affect the establishment, growth and survival of transplanted native woody and herbaceous seedlings on an abandoned agricultural field? Location: West Coast Renosterveld, Cape Floral Region, South Africa. Methods: Indigenous shrub seedlings were planted in different treatments, where either grass competition or herbivory or both were manipulated. Survival, growth and canopy cover of the seedlings were measured on a monthly basis over 14 months, and compared between treatments. Results: Experimental transplanting of indigenous shrubs into an old field showed that most of the plants investigated competed for resources with grasses on the field, and competition negatively affected the seedlings throughout the experiment. Mortality was higher, and growth was reduced for seedlings exposed to grass competition. Herbivory alone had no significant impact on the target species with the exception of Olea europaea ssp. africana. There was no significant interaction between competition and herbivory. Conclusion: Reduction of herbaceous competition significantly accelerates shrubland recovery on abandoned agricultural fields in renosterveld.

Elephant diet at the edge of the Fynbos Biome, South Africa

Reviews observations of two forest guards who from 1990 to 2000 noted the diet and preferred foods of Knysna elephants, in the Fnybos Biome along the southern coastal region of South Africa. All plants eaten by five elephants, the last three known native animals and two introduced juveniles, were noted 'this is the first evidence that elephants routinely eat the nutrient-poor, heathy shrubs and wiry grasslike plants of the fynbos'. The native animals spent 90% of their time in the forest, but when released in 1994 the introduced juveniles only spent 2 months in this habitat. From then on they spent: 80% in fynbos and rough pasture, 10% in patchy forest, 5% in mature tree plantations, and less than 5% near vegetable/croplands which they raided. 11 species of forest trees and shrubs were eaten, Rapanea, Pterocelastrus, and Ilex seemed to be preferred. The introduced juveniles mainly ate bark when in the forest. Most of the common genera of shrubs and grasslike plants of the fynbos were eaten, but most herbaceous plants were ignored. Exotic trees in plantations were occasionally damaged for their bark and many Acacia, particularly A mearnsii were broken down, and bark, pods and leaves eaten. The animals sometimes excavated and ate tubers and rhizomes of herbaceous plants such as Iridaceae and bracken fern (Pterodium) but did not eat soil. The author notes the complementary diets elephant may have with other indigenous herbivores, and discusses the commonly held view that the Knysna habitat lacks trace elements. Observations suggest elephant could be maintained in the Fynbos Biome, and their presence in this area might be useful in controlling invasive exotic plants.

Estimated spatial requirements of the medium- to large-sized mammals, according to broad habitat units, in the Cape Floristic Region, South Africa

Conservation planning in the Cape Floristic Region (CFR) of South Africa, a recognised world plant diversity hotspot, required information on the estimated spatial requirements of selected medium- to large-sized mammals within each of 102 Broad Habitat Units (BHUs) delineated according to key biophysical parameters. Spatial requirement estimates were derived for 19 omnivore and carnivore species, following a review and extrapolation of extant information. The estimates for 23 herbivore species were derived from a simple spreadsheet model based on forage availability estimates and the metabolic requirements of the species in question; this analysis incorporated adaptations of the agriculture-based Large Stock Unit (LSU) or Animal Unit approach. The LSU approach has various shortcomings but given the virtual absence of information on forage availability (quantity, quality and seasonality) for indigenous herbivores in the CFR, it provides the only extant measure of the influence of key biophysical factors on this parameter, especially at a regional scale. The outcomes of the exercise, viz. densities (hectares/animal), are presented here; these should be considered as testable hypotheses and a cautious 'management by hypothesis' approach should be adopted in their use. This information can be used to guide both conservation planning activities and practical conservation management decisions.

Potential for Biological Control of Phragmites australis in North America

Phragmites australis is a cosmopolitan plant that is undergoing a population explosion in freshwater and tidal wetlands on the east coast of North America. The rapid spread of P. australis in recent years and the virtual absence of native herbivores feeding on P. australis have led wetland ecologists to believe that either the species or more aggressive genotypes were introduced. The historical record of the occurrence of P. australis in North America and the scarcity of indigenous herbivores provide conflicting evidence for the status of the species as native or introduced. A comparison of P. australis populations from North America and other continents using advanced genetic techniques is underway to help determine the status of current and historic North American genotypes. Literature and field surveys reveal that of the 26 herbivores currently known to feed on P. australis in North America (many accidentally introduced during the last decade), only 5 are native. In Europe, over 170 herbivore species have been reported feeding on P. australis, some causing significant damage. Of these herbivores, rhizome-feeding species with considerable negative impact on P. australis performance include the lepidopterans Rhizedra lutosa (already present in North America), Phragmataecia castaneae, Chilo phragmitella, and Schoenobius gigantella. Stem-boring moths in the genera Archanara and Arenostola and the chloropid fly Platycephala planifrons can have large detrimental impacts on P. australis in Europe and should be evaluated for their potential as biological control agents. In addition, the interaction of potential control agents with accidentally introduced P. australis herbivores needs to be evaluated in North America. Regardless of the results of the genetic analyses, any decision to introduce additional host-specific herbivores in an attempt to control P. australis will require considerable dialogue. This decision needs to weigh the current negative ecological and economic impacts of P. australis and the benefits and risks of a biological control program.

Effect of Herbivore Density, Timing of Attack and Plant Community on Performance of Creeping Thistle Cirsium arvense (L.) Scop. (Asteraceae)

Creeping thistle, Cirsium arvense (L.) Scop. (Asteraceae), is one of the most serious weeds in ecological compensation areas (within-field or border refugia) in Europe. Since conventional weed control measures are restricted in compensation areas, augmenting indigenous agents for biological control of the weed may be a feasible alternative. In this paper, we studied the effect of density of the shield beetle, Cassida rubiginosa Muller (Coleoptera, Chrysomelidae), and two vegetation types typical for ecological compensation areas, on the performance of creeping thistle plants in an open field experiment. Early-season larval feeding had no measurable effect on creeping thistle growth, while late-season feeding significantly reduced shoot growth. These findings were attributed to higher feeding rates of the herbivores at higher ambient temperatures late in the season. Defoliation had a strong effect on the above-ground performance of C. arvense plants, but not on the below-ground performance. In contrast, the plant community affected all below-ground performance parameters measured, but only some of the above-ground performance parameters of creeping thistle. A combination of high levels of plant competition and herbivory by C. rubiginosa larvae led to 50% mortality in C. arvense plants during the growing season. We conclude that augmentation of indigenous herbivores of C. arvense in combination with breaking up the root system by tillage and the establishment of a highly competitive plant community of beneficial herbs may be a feasible way to control this problematic weed in ecological compensation areas.